Microporous SiO₂/Vycor membranes for gas separation

In this study, porous Vycor tubes with 40 Å initial pore diameter were modified using low pressure chemical vapor deposition (LPCVD) of SiO₂. Diethylsilane (DES) in conjunction with O₂ or N₂O were used as precursors to synthesize the SiO₂ films. Both “single side” (reactants flowing on the same side of porous membrane) and “counterflow” (reactants flowing on both sides of porous membrane) reactant geometries have been investigated. The flow of H₂, He, N₂, Ar, and toluene (C₇H₈) was monitored in situ after each deposition period. Membranes modified by the “single side” reactants geometry exhibited good selectivities between small and large molecules. However, cracking in these membranes after prolonged deposition limited the maximum achievable selectivity values. Higher selectivities and better mechanical stability were achieved with membranes produced using the “counterflow” reactants geometry. Pore narrowing rate was observed to increase with oxidant flow (O₂ or N₂O). For membranes prepared using both oxidants, selectivities on the order of 1000: 1 were readily attained for H₂ and He over N₂, Ar, and C₇H₈. As compared to O₂, the use of N₂O caused improvements in both the pore narrowing rate and N₂: C₇H₈ selectivity. Membranes prepared using the “counterflow” geometry showed no signs of degradation or cracking after thermal cycling.